298 



STOMACH AND INTESTINE. 



projecting into its cavity. Its interior surface 

 is folded and villous. The whole structure 

 appears to be connected with a kind of chy- 

 lous absorption. 



The alimentary canal of the Epizoa differs 

 from that of the cavitary Entozaa, in being 

 generally surrounded by a glandular mass, the 

 function of which is probably hepatic. The 

 Cirripeda have prehensile jaws, and a ter- 

 minal mouth and anus. In some, the canal 

 has a gastric dilatation. Hepatic follicles, 

 similar to those already described, occur here 

 also. And St. Ange and Serres have found a 

 tube analogous to the typhlosole. 



The digestive tube of the Crustacea may 

 be reduced to two chief forms, which corre- 

 spond with other differences in the nature and 

 structure of their possessors. Thus in those 

 lower Crustaceans which are suctorial and para- 

 sitic, the canal is a very simple one. A proboscis 

 conceals a pair of lancet teeth, and is followed 

 by a straight intestine, around which are 

 clustered a dense mass of follicles, supposed 

 to constitute a liver. The higher Crustaceans 

 possess a complicated apparatus of forceps 

 and jaws. A short oesophagus leads to a 

 large spherical cavity, which occupies the head 

 of the animal, and which, although sometimes 

 called a stomach, contains hard structures 

 that render it analogous to a gizzard. A well- 

 marked constriction separates this organ from 

 the intestine, which is sometimes simple and 

 nearly straight, sometimes divided into two 

 portions distinguished by a projecting valve. 

 The liver is conglomerate, and divided into 

 lobules. Rarely, one or two caecal tubes are 

 also present. 



The alimentary canal of the Insect offers 

 what are rather varieties of development 

 than any regular transition, such as we have 

 remarked in some of the preceding orders: 

 varieties which the metamorphosis of the larva 

 at present seems to complicate instead of ex- 

 plain. 



In the larva, the canal is comparatively 

 simple, and somewhat approaches the condi- 

 tion seen in the lower Annelida : being a straight 

 tube, with a mouth and anus at opposite ends 

 of the body. In many Hymenopterous larvae, 

 the latter aperture is absent, In others it is 

 only developed towards the end of this stage 

 of life, when an excrement or meconium as 

 we may perhaps call it is for the first time 

 expelled. But though such an intestine 

 might seem to resemble that of the anen- 

 terous Infusory, or the hydriform Polyp, we 

 must recollect that it differs from these in the 

 important fact of its not being used for the 

 double purpose of ingestion and egestion. 

 The complications of the above simple canal 

 relate chiefly to its subdivision, and to the addi- 

 tion of blind tubes. A gastric dilatation is the 

 first to appear ; its extremities then become 

 constricted, and its calibre enlarged. An oeso- 

 phagus, a crop or ingluvies, a small and a large 

 intestine, may also be added. Sometimes the 

 supposed stomach is transversely divided into 

 two cavities, and complicated by short caeca. 

 In other instances, longer tubes open into the 



same part of the canal. While in others, they 

 open into the intestine below this point; and 

 are hence presumed to be biliary. 



In the perfect Insect the varieties of form 

 are still more numerous and perplexing. 

 Besides the complicated prehensile and dental 

 apparatus, there is often an oesophagus, a crop, a 

 muscular gizzard, a stomach, a small intestine, 

 a large intestine, and a narrower rectum. But 

 development is manifested, not only by differ- 

 ences in the diameter and structure of dif- 

 ferent lengths of the tube itself, but also by its 

 complication, through the addition of supple- 

 mentary organs of a more or less tubular form. 



The ingluvies or crop is present in many 

 but not all of the suctorial genera. It is 

 sometimes distinctly glandular. And even 

 where, as in the Bee, this character is less 

 prominent, it is still probably a secreting organ. 

 But its uses seem to be mainly those of accu- 

 mulation. 



The gizzard is generally added to the former 

 organ. It is characterized by distinct mus- 

 cularity, and a more or less hard or horny 

 epithelium, which is often developed into 

 plates, protuberances, hairs, or teeth. Some- 

 times it is only rudimentary: a toothed 

 oesophagus subserving its functions in some 

 insects ; while in others, it is reduced to a 

 mere thickening of the muscular wall of this 

 part of the canal. 



The stomach is also of various form and 

 size. In some insects it is simple ; in others 

 it is more or less plicated or cellulated, or 

 its cells are even prolonged into short caeca. 



The peculiarities of the remaining subdivi- 

 sions of the canal are chiefly those of their 

 length and width, and in the degrees of con- 

 striction by which they are separated from each 

 other. As yet, however, it has been found im- 

 possible to make out any intimate connection 

 between these differences in the anatomy of 

 the tube and the habits of the animal possess- 

 ing it. Indeed, the general relations of this 

 kind seen in other orders often seem to be 

 interrupted or even reversed in the insects. 



The numerous tubes which open into 

 the intestinal canal present still more diver- 

 sity. They are often named salivary, biliary, 

 or urinary organs. Thus those tubes which 

 open into the earlier part of the intestinal 

 cavity are called salivary ; those which empty 

 themselves into the commencement of the small 

 intestine are regarded as biliary; and, finally, 

 those which open into the canal at or near its 

 termination, are considered urinary. It is 

 only the first of these that, after many grada- 

 tions, fairly attain the glandular development 

 which a conglomerate condition implies. The 

 second vary chiefly in number, and in the fre- 

 quency of their anastomosis. The third 

 are rarely vesicular in shape. 



The digestive canal of the Arachnida offers, 

 on the whole, more uniformity. The chief 

 divisions of this order are the parasitic, the 

 spiders, and the scorpions. All are " carni- 

 vorous :" a term which here, as often else- 

 where, is only approximative^ correct; since 

 most of them do not devour the flesh, but 



